Implementing an asynchronous iterator

Per PEP-492 I am trying to implement an asynchronous iterator, such that I can do e.g.

async for foo in bar:
    ...

Here is a trivial example, similar to the one in the docs, with a very basic test of instantiation and async iteration:

import pytest

class TestImplementation:
    def __aiter__(self):
        return self
    async def __anext__(self):
        raise StopAsyncIteration


@pytest.mark.asyncio  # note use of pytest-asyncio marker
async def test_async_for():
    async for _ in TestImplementation():
        pass

However, when I execute my test suite, I see:

=================================== FAILURES ===================================
________________________________ test_async_for ________________________________

    @pytest.mark.asyncio
    async def test_async_for():
>       async for _ in TestImplementation():
E       TypeError: 'async for' received an invalid object from __aiter__: TestImplementation

...: TypeError
===================== 1 failed, ... passed in 2.89 seconds ======================

Why does my TestImplementation appear to be invalid? As far as I can tell it meets the protocol:

1. An object must implement an __aiter__ method ... returning an asynchronous iterator object.
2. An asynchronous iterator object must implement an __anext__ method ... returning an awaitable.
3. To stop iteration __anext__ must raise a StopAsyncIteration exception.